A liquid crystal display panel has been developed which has a pixel portion and a driver circuit integrated over the same glass substrate by using thin film transistors (TFTs) formed of a crystalline semiconductor film with a thickness of several ten nanometers. Further, a technique to manufacture a central processing unit (CPU) which is a central function of a computer by using high functional polycrystalline silicon which is superior in crystallinity has been reported as well (for example, refer to Non-patent Documents 1 and 2). In the case where a pixel portion and a driver circuit are integrated over the same glass substrate, a memory is also formed over the same substrate. With such a technical progress, a technique called System on Panel to integrate a display function and a computer function realized by a CPU over a glass substrate is becoming to be realized.
A memory is generally formed by using a silicon wafer. However, the aforementioned system panel which is required to be thin and lightweight is expected to have a memory formed by using a plastic substrate or a plastic film substrate. This is because plastic is low in specific gravity, lightweight, and high in shock resistance as compared to a silicon wafer and glass.
However, a plastic material is generally poor in heat resistance, thus the highest temperature of the process is required to be decreased. Accordingly, a crystalline semiconductor film to form a memory cannot be formed.
[Non-Patent Document 1]
Imaya, A., “CG Silicon technology and its application”, AM-LCD 2003 Digest, p. 1, 2003.
[Non-Patent Document 2]
Lee, B. Y., et al., “A CPU on a glass substrate using CG-Silicon TFTs”, ISSCC Digest, p. 164, 2003.